[1102.0800] Measuring the cosmological bulk flow using the peculiar velocities of supernovae

Authors:

De-Chang Dai, William H. Kinney, Dejan Stojkovic

Abstract:

We study large-scale coherent motion in our universe using the existing Type
IA supernovae data. If the recently observed bulk flow is real, then some
imprint must be left on supernovae motion. We run a series of Monte Carlo
Markov Chain runs in various redshift bins and find a sharp contrast between
the z < 0.05 and z > 0.05 data. The$z < 0.05 data are consistent with the bulk
flow in the direction (l,b)=({290^{+39}_{-31}}^{\circ},
{20^{+32}_{-32}}^{\circ}) with a magnitude of v_bulk = 188^{+119}_{-103} km/s
at 68% confidence. The significance of detection (compared to the null
hypothesis) is 95%. In contrast, z > 0.05 data (which contains 425 of the 557
supernovae in the Union2 data set) show no evidence for bulk flow. While the
direction of the bulk flow agrees very well with previous studies, the
magnitude is significantly smaller. For example, the Kashlinsky, et al.'s
original bulk flow result of v_bulk > 600 km/s is inconsistent with our
analysis at greater than 99.7% confidence level. Furthermore, our best-fit bulk
flow velocity is consistent with the expectation for the \Lambda CDM model,
which lies inside the 68% confidence limit.

There have been studies arguing that there is an anomalously large bulk flow in the universe, up to 600 km/s to 1000 km/s on scales extending up to 800 Mpc. (Other studies have claimed a somewhat smaller flow.) A detection has been argued from both the kinetic SZ effect and peculiar velocities of galaxies, see http://cosmocoffee.info/viewtopic.php?t=1227&highlight= for one of the latter papers.

One possibly problematic aspect of the peculiar velocity studies is the use of composite maps which have been put together from different observations with different systematics. I recently mentioned ( http://cosmocoffee.info/viewtopic.php?t=1734&highlight= ) a study where data from a single nearby catalogue of Tully-Fisher distances is analysed (and no large bulk flow is seen).

The present paper uses Union2 supernovae, which have the advantage that the systematics are better understood and the distances are less susceptible to poorly known astrophysics. The authors perturb the redshift and luminosity distance with a single bulk peculiar velocity, and fit this parameter to the data.

For [tex]z<0.05[/tex] the authors find a bulk flow with a direction consistent with the previous works, but a much smaller amplitude, [tex]188^{+119}_{-103}[/tex] km/s. They note that for this redshift range the significance compared to the null case os 95\%, but it gets progressively worse as more high redshift data is included. For [tex]z>0.05[/tex] there is no detection at all. The amplitude is consistent with [tex]\Lambda[/tex]CDM and inconsistent with the paper discussed at http://cosmocoffee.info/viewtopic.php?t=1227&highlight= at 99.7\% C.L..

First, I must say this paper looks like a draft version to me. The plots are a little bit odd. Projecting the whole sky on a rectangle does not help very much: you cannot really infer the actual distribution of the datapoints on the celestial sphere (which may be crucial if the sample is inhomogeneous!). Also, I don't understand what's the point in doubling the likelihood/probability curves by plotting -ln L below L?
Anyway, note how small is the z<0.05 sample and how nonuniform is the z>0.05 one (see the arc in Fig. 5?). So, I wouldn't say the high redshift subsample can tell you much. It's not just by chance that the likelihood blob has its maximum somewhere close to this arc, because this is the part of the sky that most of the data come from. As for the low-redshift subsample, indeed no large bulk flow is observed; however, the direction is quite consistent with other findings, which makes me think that possibly some systematics when converting from the 'signal' to 'velocity' (in km/s) may be at fault (not necessarily here, but in general).

Maciej Bilicki wrote:As for the low-redshift subsample, indeed no large bulk flow is observed; however, the direction is quite consistent with other findings, which makes me think that possibly some systematics when converting from the 'signal' to 'velocity' (in km/s) may be at fault (not necessarily here, but in general).

What kind of a systematic do you have in mind? (When it comes to the supernovae, the effect of the velocity is very straightforward.)

Maciej Bilicki wrote:First, I must say this paper looks like a draft version to me. The plots are a little bit odd. Projecting the whole sky on a rectangle does not help very much: you cannot really infer the actual distribution of the datapoints on the celestial sphere (which may be crucial if the sample is inhomogeneous!). Also, I don't understand what's the point in doubling the likelihood/probability curves by plotting -ln L below L?

Sorry you don't like the figures.

Anyway, note how small is the z<0.05 sample and how nonuniform is the z>0.05 one (see the arc in Fig. 5?). So, I wouldn't say the high redshift subsample can tell you much. It's not just by chance that the likelihood blob has its maximum somewhere close to this arc, because this is the part of the sky that most of the data come from.

I don't understand: the best-fit direction in the z>0.05 sample is not statistically significant, so where the "blob" is on the sky is totally irrelevant. We did worry about the effect of the sample inhomogeneity induced by the SDSS slice (the arc you see), but found no evidence that it introduces a significant bias. In fact, if you take the entire Union2 data set as a whole, the best-fit direction is pretty much the same as if you take the z<0.05 sample alone. The only change is that the significance of the detection is reduced. The z>0.05 data just adds noise, as far as we can tell, despite being the majority of the Union2 data.

Maciej Bilicki wrote:As for the low-redshift subsample, indeed no large bulk flow is observed; however, the direction is quite consistent with other findings, which makes me think that possibly some systematics when converting from the 'signal' to 'velocity' (in km/s) may be at fault (not necessarily here, but in general).

What kind of a systematic do you have in mind? (When it comes to the supernovae, the effect of the velocity is very straightforward.)

I'm not sure what the systematic might be either.

It is true that dipole subtraction can introduce a systematic for nearby supernovae (see Davis et al.), but I do not believe this is an issue, for two reasons:
(1) The effect would introduce a systematic which would bias you opposite the CMB dipole, i.e. opposite the direction we actually see the bulk flow.
(2) Davis et al. conclude the bias is only significant for supernovae redshifts z<0.02. The lowest redshift supernova in the Union2 data set is at z=0.015, and there are only 32 supernovae with redshift z<0.02. I would be very surprised if any bias from dipole subtraction were significant compared to other sources of noise in the sample.

I just think that plotting datapoints on the sky in some Aitoff projection would give a clearer picture :)

Anyway, note how small is the z<0.05 sample and how nonuniform is the z>0.05 one (see the arc in Fig. 5?). So, I wouldn't say the high redshift subsample can tell you much. It's not just by chance that the likelihood blob has its maximum somewhere close to this arc, because this is the part of the sky that most of the data come from.

I don't understand: the best-fit direction in the z>0.05 sample is not statistically significant, so where the "blob" is on the sky is totally irrelevant.

OK, I didn't get it right. Indeed the 99% area covers some half of the sky.

We did worry about the effect of the sample inhomogeneity induced by the SDSS slice (the arc you see), but found no evidence that it introduces a significant bias. In fact, if you take the entire Union2 data set as a whole, the best-fit direction is pretty much the same as if you take the z<0.05 sample alone. The only change is that the significance of the detection is reduced. The z>0.05 data just adds noise, as far as we can tell, despite being the majority of the Union2 data.

What's the reason for this? Could this be because of large errors of the high-redshift data?
Still, it is interesting to see a discrepancy between your bulk flow magnitude of low-redshift SNe and that of e.g. Watkins et al. (2009), who also used SNeIa - 105 of them limited by 150 Mpc/h from Tonry et al. (2003). And they get a bulk flow of >400 km/s from SNe alone... (their table 1)

Maciej Bilicki wrote:Still, it is interesting to see a discrepancy between your bulk flow magnitude of low-redshift SNe and that of e.g. Watkins et al. (2009), who also used SNeIa - 105 of them limited by 150 Mpc/h from Tonry et al. (2003). And they get a bulk flow of >400 km/s from SNe alone... (their table 1)

Syksy Rasanen wrote:
What kind of a systematic do you have in mind? (When it comes to the supernovae, the effect of the velocity is very straightforward.)

Well, I don't know what these systematics may be. Still, somebody must be doing something wrong, if measurements of the amplitude are inconsistent, although directions agree more or less. In particular, it would be very strange if e.g. clusters had a much faster bulk flow than SNe Ia...

I guess you're familiar with the issue of Lauer & Postman's (1994) results. They found a bulk flow direction completely at odds with other estimates. Now we know that their distance indicators, BCGs, where at fault. Currently it's different, because the distances (hence peculiar velocities) are measured with a much better accuracy (and much more trustworthy I would say).

Maciej Bilicki wrote:Still, it is interesting to see a discrepancy between your bulk flow magnitude of low-redshift SNe and that of e.g. Watkins et al. (2009), who also used SNeIa - 105 of them limited by 150 Mpc/h from Tonry et al. (2003). And they get a bulk flow of >400 km/s from SNe alone... (their table 1)